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Ian Dobinson has recently asked for my input on 70-200 f/2.8 IS MkII + 2xTC MkIII autofocus performance, in particular relative to 100-400.
In order not to go by memory alone, today I\'ve repeated a few little tests with the combo in a garden type setting.
My assessment is as follows. The combo is somewhat more sluggish than 100-400. In addition, it does initiate a limited focus search on low and medium contrast targets probably a bit more often than 100-400. However, 100-400 has a tendency to do a full travel focus search more often which takes longer time.
In reasonably good light and on contrasty targets, the combo\'s AF speed is quite adequate even on the full focusing distance limiter setting.
Haven\'t tried using the combo on BIF, but my feeling is that 100-400 would outperform it although not by much. In effect, neither 100-400 nor the subject combo are BIF champions, but they could be of service.
The camera used was 7D, which probably has a pretty good extreme defocus handling which also helps with AF swings and focus search.
A few words about IQ. My 100-400 has a better IQ by a fair bit.....my combo is somewhat soft wide open, and stopping down makes no big difference. I suppose the combo can result in OK images if sharpened and postprocessed carefully and well.
The bottom line: my 70-200 f/2.8 IS MkII + 2xTC MkIII combo is not the best substitute for my 100-400, in fact I\'d use that combo only if I had nothing else at my disposal.
The test images below are SOOC JPGs resized for web only.